1. Field of the Invention
The present invention relates to an intermediate transfer belt which is used for an image forming apparatus of an electrophotographic system, and an image forming apparatus including the intermediate transfer belt.
2. Description of the Related Art
Conventionally, an image forming apparatus of an electrophotographic system (a copier, a laser beam printer, etc.) uses an intermediate transfer belt. The intermediate transfer belt is a belt for transferring a toner image held on a photoconductor drum, etc., onto paper, etc. For a scheme for achieving a further improvement in image quality, there is a technique in which an elastic layer is provided to an intermediate transfer belt.
By the presence of the elastic layer in the intermediate transfer belt, stress occurring due to contact of thick paper (e.g., paper with a thickness of the order of 300 μm), etc., is distributed, and stress on a toner image is reduced and the followability to a rough texture of a transfer material is improved. By this, transfer efficiency can be improved. For a method of providing elasticity to the intermediate transfer belt, in general, rubber materials (NBR, CR, ECO, etc.) are often used.
However, the rubber materials have high surface adhesion and also have a low wear resistance. Thus, measures need to be taken against the high surface adhesion and the low wear resistance. As one method for the measures, there is known a technique for forming, on a soft elastic layer, a surface layer which is harder than the elastic layer. For example, JP 11-84890 A, JP 8-160760 A, and JP 2011-138031 A propose that a surface layer higher in hardness than rubber is formed on an elastic layer (rubber layer) to provide a release function.
However, According to the above-described technique, since the surface layer is still too soft, when the intermediate transfer belt is durably used, the surface layer is abraded due to friction of contact members, causing a problem of loss of the release function. To solve this problem, an increase in the hardness of the surface layer is considered. As a result, abrasion of the surface layer is suppressed, but since the elastic layer is soft, another problem occurs. Specifically, although the wear resistance improves by increasing the hardness of the surface layer, as shown in FIG. 4, deformation (stress concentration) of the surface layer caused by edge portions of thick paper when the thick paper passes through cannot be borne. As a result, cracking occurs in the surface layer.
In many image forming apparatuses that use an intermediate transfer belt, a belt drive roller that stretches and drives the intermediate transfer belt is used as a secondary transfer counter roller. Hence, in a secondary transfer portion, a surface layer provided on a surface of the intermediate transfer belt is subjected to deformation in a thickness direction of the intermediate transfer belt caused by edges of thick paper, while the surface layer extends in a circumferential direction of the intermediate transfer belt. This leads to a disadvantageous outcome in terms of the suppression of cracking of the surface layer.